1. Field of the Invention
This invention relates to a disk used as an optical information recording medium, a disk recording apparatus for recording data on the disk, and a disk reproducing apparatus for reproducing data from the disk.
2. Description of the Prior Art
As a recording medium capable of high density recording of data, an optical disk is known. As one method of further improving the recording density of the optical disk, such a method is considered that the gap between each adjacent signal recording tracks is narrowed in order to improve the recording density in the radial direction of the disk. In this case, however, if a reading laser beam used during reproduction does not have a satisfactorily small spot diameter, narrowing the gap between adjacent recording tracks simply results in signals of the tracks to a targeted track being readout in addition to the signal of the targeted track, thus causing a problem in that unnecessary crosstalk components are mixed into a reproduced signal at the signal detection stage. The existence of crosstalk causes an inter-code interference between signal tracks, resulting in an increase in error rate of a reproduced signal. As a result, an attempt has been made to electrically cancel any crosstalk component developed through the optical reproducing system to improve the quality of a reproduced signal. For example, a method disclosed in Japanese Laid-Open Patent Application No. 3-40225 solves such a problem by presenting a reproduction method using a plurality of laser beams. That is, other than the laser beam for reproducing the targeted track, a plurality of laser beams are additionally provided for reproducing the track adjacent thereto in order to obtain crosstalk signals. These plural laser beams reproduce signals independently and then, the signals of the adjacent tracks are electrically subtracted from the signal of the targeted track including crosstalk signals to thereby reduce the crosstalk component.
In addition, as one method of recording signals on a disk, a constant angular velocity (CAV) system is known, in which a signal is recorded on or reproduced from a disk while the disk is being rotated at a constant angular velocity. In general, recording is made while controlling the rotation of the disk at a constant rotational speed by phase-synchronizing a spindle motor with the reference clock synchronized with the recording data.
However, with the method disclosed in Japanese Laid-Open Patent Application No. 3-40225, other than the laser beam for reproducing a targeted track, two laser beams and optical detectors are respectively provided for reproducing the adjacent tracks thereto, requiring three optical reproducing systems in total, which means that becomes complex and the operation difficult compared with the system using one laser beam for reproduction. In addition, other than the optical system, three signal processing system each including a pre-amplifier, an A/D converter, a digital filter and the like are required, thus causing an increase in production costs.
Here, the signals of the track adjacent to the targeted track are respective signals reproduced one rotation before and one rotation after the targeted signal when rotating the disk for continuous reproduction. As a result, it is theoretically possible to obtain the signals the adjacent tracks using two signal delay circuits each delaying a signal for one rotation of the disk. However, with a conventional disk, it was extremely difficult to obtain signals of one rotation before and one rotation after the targeted signal. This is because even with the CAV system having the rotation controlled at a constant rotational speed by a servo circuit, a jitter or unevenness of rotation, called wow flutter, due to unevenness in torque of the spindle motor, eccentricity of gravity of the disk and the like can unavoidably develop. As a result, even if a signal having a stable time axis is recorded directly on a disk having an unevenness in rotation, the disk to be obtained will be unstable in the number of clocks of the recorded signal for one rotation of the disk and the clock phase of the recorded signal. FIG. 5 shows an example the signal arrangement of the conventional disk shown above. From a reproduced signal of such a conventional disk, accurate signals of the tracks adjacent to the targeted track could not be obtained even if signal delay circuits each for one rotation of the disk are employed.
Therefore, with a conventional disk recording apparatus, signal data cannot be recorded on the disk so that the recorded signals of the tracks adjacent to the targeted track can be closely coincided in phase with each other, and with a conventional disk reproducing apparatus, it is very difficult to obtain accurate signals of the adjacent tracks.